Heat exchangers that utilize a single-phase working fluid to transfer heat from a heat source or to a heat sink are known as single-phase heat exchangers. Single-phase heat exchangers are used in a variety of applications ranging from radiators of conventional automobiles to more exotic water-to-ammonia heat exchangers for sustaining life in outer space, e.g., aboard a space shuttle or a space station. Single-phase heat exchangers are also used in other diverse applications, such as removing waste heat from electronic devices, e.g., microprocessors, cooling fusion reactor diverters and producing slush hydrogen.
Compact single-phase heat exchangers are particularly desirable in applications having relatively high heat fluxes. For example, the continually increasing speeds and complexity of microprocessors cause these microprocessors to generate commensurately increasing amounts of heat. Present generation microprocessors typically have heat fluxes in the range of 5 watts/cm2 to 15 watts/cm2. The next several generations of microprocessors are predicted to have much greater heat fluxes, e.g., on the order of 50 watts/cm2 to 200 watts/cm2 or more. One type of compact heat exchanger contemplated for high flux heat transfer applications is what has become known as a normal-flow heat exchanger (NFHX). Specific embodiments of NFHXs have been previously disclosed by the present inventor, e.g., in U.S. Pat. Nos. 5,029,638 and 5,145,001. An NFHX is desirable for applications such as microprocessor cooling because it provides: (1) a single phase heat exchanger having a high surface heat flux capability; (2) a compact heat exchanger in which the working fluid experiences a generally small pressure drop as it passes through the heat exchanger; and (3) a small and lightweight heat exchanger having a high thermal transfer efficiency.
FIG. 1 shows one embodiment of an NFHX 20 as taught in the aforementioned patents. NFHX 20 includes a heat-transfer surface 22 for thermally communicating with a heat source or sink (not shown). For example, heat-transfer surface 22 may be thermally coupled to a microprocessor for removing waste heat from the microprocessor. NFHX 20 further includes a heat-transfer element 24 comprising an inlet end 26 located opposite heat-transfer surface 22 and a plurality of closely spaced plates 28. The spaces between plates 28 define a plurality of passageways 30 that are substantially normal to heat-transfer surface 22. A plurality of outlet manifolds 32 are located adjacent, and parallel, to heat-transfer surface 22. Outlet manifolds 32 are spaced from one another and each intersects each passageway 30 to permit a working fluid 34 to flow from the passageways into the outlet manifolds.
During use, working fluid 34 flows from a source (not shown) into passageways 30 via inlet end 26, and then through passageways 30 to outlet manifolds 32 in a direction substantially normal to heat-transfer surface 22, and out of heat-transfer element 24 through the outlet manifolds in a direction substantially parallel to the heat-transfer surface. As working fluid 34 flows through heat-transfer element 24, it gains, or loses, most of its heat while flowing through passageways 30. The heat is transferred to, or from, working fluid 34 via plates 28, which are in thermal communication with heat-transfer surface 22. It is the fact that the majority of heat transfer to or from working fluid 34 occurs in passageways 30 that are normal to heat-transfer surface 22 that NFHX 20 gets its name.
Unfortunately, conventional NFHXs have a number of shortcomings. For example, for NFHX 20 to provide highly efficient heat removal, the spacing between plates 28, and, hence the depth of normal-flow passageways 30, must be very small, preferably less than 0.10 mm. Relatedly, to increase the heat transfer efficiency of NFHX 20, it is desirable to make plates 28 relatively thin, e.g., on the order of 0.25 mm or less. Accordingly, the dimensions of plates 28, passageways 30 and outlet manifolds 32 must be precisely controlled. Typically, the elements of an NFHX are fabricated using techniques such as traveling-wire EDM and traditional machining. However, these techniques are typically not capable of, or are impractical for, forming elements having the small dimensions necessary for producing NFHXs capable of handling the high heat fluxes of the next generation of microprocessors.
In addition, the configuration of passageway 30 and outlet manifolds 32 produces performance limitations that limit conventional NFHX 20 from achieving the needed high flux heat transfer capacity. For example, the location of outlet manifolds 32 proximate to heat-transfer surface 22 interferes with the direct conduction of heat between the heat-transfer surface and plates 28. Outlet manifolds 32 extend the length of NFHX 20 and, therefore, interrupt thermal conduction from heat-transfer surface 22 to plates 28 along the entire length of the NFHX. This interruption increases the thermal resistance between heat-transfer surface 22 and working fluid 34 in passageways 30. For this reason, it is desired to make the outlet manifolds 32 as small as possible in cross-sectional dimension, but this has the detrimental effect of increasing the pressure drop therewithin. In addition, pressure drop within outlet manifolds 32 creates non-uniform flow distribution of working fluid 34 over the cooling area of plates 28, thereby reducing the heat transfer efficiency of NFHX 20.
The normal-flow heat exchanger of the present invention, however, overcomes these and other shortcomings of conventional normal-flow heat exchangers.